The present disclosure relates to implantable prosthetic heart valves. More particularly, it relates to prosthetic heart valves incorporating a stent and methods of manufacture thereof.
Various types and configurations of prosthetic heart valves are used to replace diseased natural human heart valves. The actual shape and configuration of any particularly prosthetic heart valve is dependent to some extent upon the valve being replaced (i.e., mitral valve, tricuspid valve, aortic valve, or pulmonary valve). In general, the prosthetic heart valve designs attempt to replicate the function of the valve being replaced and thus will include valve leaflet-like structures used with either bioprosthesis or mechanical heart valves prosthesis.
As used throughout the specification, a “prosthetic heart valve” is intended to encompass bioprosthetic heart valves having leaflets made of a biological material (e.g., harvested porcine valve leaflets, or bovine or equine pericardial leaflets), along with synthetic leaflet materials or other materials. Bioprosthetic valves are divided into two broadly defined classes; namely, stentless and stented prosthetic heart valves. Stentless bioprosthetic heart valves do not have a support frame. Rather, the biological valve member is sutured to a flexible cloth material. The hemodynamics of a stentless valve may more closely approximate that of a natural heart valve. A drawback of a stentless valve, however, is that it is more difficult to implant into the patient than a stented valve. Furthermore, a stentless valve can be collapsed and deformed by the action of the heart because it has no support structure. The action of the heart muscles on this type of valve can fold the valve material and create unexpected stress risers that can eventually lead to failure.
Stented bioprosthetic heart valves have a frame (or stent) to which the biological valve material is attached. The biological valve members are sutured to the stent that provides support for the valve member in the patient's body. The stent prevents the biological valve members from collapsing and simplifies the insertion of the valve into the annulus of the patient after excision of the diseased valve. The stented bioprosthetic valve imitates the natural action of heart valves and provides a structure that is relatively compatible with the cardiovascular system. Stented prosthetic heart valves are believed to have important clinical advantages over mechanical or non-tissue prosthetic valves.
Known stent constructions for stented bioprosthetic heart valves comprise two or three support structures, commonly referred to as stent posts or commissure posts, projecting from a base frame or ring. The stent or commissure posts define the juncture between adjacent tissue or synthetic leaflets otherwise secured thereto, and are typically linear in projection from the base frame. With some stent configurations, the stent posts are provided by a stent post frame that in turn is assembled to the base frame, with the two components collectively providing the completed stent. The shape of the stent post frame is generated by a continuous support rail or wire (sometimes referred to as a “wireform”) that is made of either a steel alloy or thermoplastic material, and a plastic wall. The support rail typically is circular in cross-section, and formats the stent post frame shape to interpose the stent posts between lower cusp portions. The base frame generally conforms to the shape of the stent post frame for attachment to the cusp portions, and provides rigid support in the lateral direction. A covering of porous biocompatible cloth is fitted about the stent, completely enclosing the stent post frame and the base frame. The cloth cover provides a sewing attachment point for the leaflet commissures and cusps. In some constructions, a cloth-covered suture ring or sewing cuff can be attached to the cloth-covered stent for sewing of the prosthetic valve within the patient's heart.
Most currently-available prosthetic heart valve stents (and in particular the stent post frame thereof) are geometrically formed from a devolved cylinder. That is to say, a right circular cylinder cut in such a way as to have a plurality of stent post tips adjacent to some parabolic/elliptic scallops, forming the leaflet attachment or margin of attachment. As a consequence of this design, the cross-section of any horizontal plane changes from right rectangular to arc-like, resulting in a very high non-linear increase in cross-sectional inertia as well as eccentricity of the neutral axis from the stent post tip to the cusp of the leaflet. The structural implications of this are two-fold. Firstly, there is considerably more radial rigidity in the lower sections of the stent as compared to the upper sections, and as a direct consequence of the rigidity and neutral axis eccentricity, the lower sections will be considerably more stressed than the upper sections. This structural configuration also represents a challenge for the leaflets, as the top of the stent post frame is allowed to deflect more radially than the base, and as a result the leaflets have a very non-linear distribution of radial deflection, and hence demand, from the stent post downwardly.
The manner in which the stent post frame is attached to the base frame, as well as attachment of the leaflets to the collective stent, can also problematically affect responsiveness of the stented prosthetic heart valve to various forces following implant. For example, if the stent post frame is rigidly connected to, and thus rigidly constrained by, the base frame, a considerable concentration of demand is exerted at the junction of the stent post frame with the base frame.
In light of the above, a need exists for stented prosthetic heart valve exhibiting uniform stress demand and a smooth, continuous bend of the linear stent post.